Graphene has unique optoelectronic properties that result in a variety of potential photonic applications, such as optical modulators, plasmonic devices and terahertz (THz) emitters. Particularly promising is terahertz (THz) photodetection, in which graphene-based devices may offer significant advantages over existing technology in terms of speed and sensitivity.
Due to graphene's low electronic heat capacity and relatively large electron-electron relaxation rate compared to its electron-phonon relaxation rate, hot electron effects are prominent in graphene even at room temperatures. The hot electron effects have been exploited to attain fast and sensitive THz detection via the photothermoelectric effect and bolometric effect.
However a significant challenge remains in increasing graphene's absorption. Graphene's interband absorption is determined through a frequency-independent constant πα≈2.3%, where α is the fine structure constant. Owing to its zero band gap nature, doped graphene shows a relatively high DC conductivity, resulting in a considerable Drude absorption (free carrier response) in the THz range. However, the Drude absorption in graphene is strongly frequency dependent, decreasing as (ωτ)−2 at high frequencies ω>>1/τ, where τ is the scattering time, proportional to graphene's mobility, and typically ranges between 10 fs and 100 fs in graphene. Thus, the Drude absorption rolls off at lower frequencies in higher mobility (higher τ) graphene samples.
A number of efforts have been made to increase the absorption in graphene photodetectors.
For example, quantum dots have been deposited on graphene to enhance the light-matter interaction. However, this approach is limited to the visible or near infrared (where the interband absorption of the quantum dot lies), and the response times are unacceptably slow.
Another approach contemplates placing of the detector in a microcavity, which resonates at a selected frequency. This approach can enhance absorption, but to date this has been demonstrated only at near-infrared wavelengths, and can be cumbersome for long wavelength THz radiation.
Coupling the detector to an antenna is a viable approach for frequencies up to the low THz, but there are few demonstrations of antenna-coupled graphene devices, and the approach is applicable only to devices whose size is much smaller than the wavelength.
It is therefore desirable to overcome the deficiency of the prior approaches in pursuit to achieve a strong absorption and attain improved operational parameters in graphene-based detectors, specifically, through enhanced plasmon resonances in finite-width graphene.